sin(π/10)x sin (13π/10)
=-1/2[cos(π/10+13π/10)-cos(π/10-13π/10)]
=-1/2[cos(7π/5)-cos(-6π/5)]
=-1/2[cos(π+2π/5)-cos(-π-π/5)]
=-1/2[-cos2π/5+cosπ/5]